Monday, April 22, 2013

What is String args is used for or Why we use String arg[] in main method
is a common doubt among Java beginners. First thing newbies exposed while
learning Java programming language is writing HelloWorldprogram in Java, Though HelloWorld is
smallest sort of program, it contains lot of useful information and concept for
newbies Java programmers, one of them is main method and it's different
attribute. Though I covered bit of String args[], while discussing
Why
main is public static in Java, I thought to write this article to explain
String args array in more detail. Let's see a simple Java HelloWorld again to
understand more about String args in Java.

String args[], which is actually an array
of java.lang.String type, and it's name is args here. It's
not necessary to name it args always, you can name it strArray or
whatever you like, but most programmer prefer to name it args, because
that's what everyone else do. When we run
a Java program from command prompt, we can pass some input to our Java
program. Those inputs are stored in this String args array. For example if we
modify our program to print content of String args[], as shown
below:

and after compiling, we run this Java program asjavaStringArgsTestJava J2EE
Android, then this program will print following output :

String arguments passed while running this Java
Program :

Java

J2EE

Android

because, we have passed three arguments separated by white space. If you
want to combine multiple words, which has some white space in between, then you
can enclose them in double quotes. If we run our program again with following
command :

javaStringArgsTest"This
is one argument" "This is another argument"

it will print:

String arguments passed while running this Java
Program :

This is one argument

This is another argument

By the way, if you are using Eclipse to run Java Program, then you can
provide String argument to your Java program using "Run
Configuration". When you run Java program, by right click on Java
class with main
method, it creates a Run Argument for that class. you can access them as Right
click-->Run As-->Run Configuration. In second tab, you can see two
section Program Arguments and VM Arguments. You can pass all your String
arguments from this program arguments section. As shown in this image

By the way, you can write your String args[] as String[]
args as well, because both are valid way to declare String
array in Java. Another interesting thing to note is that, from Java 5
onwards, you can varargs
or variable arguments in main method instead of String
args[]. Which means following declaration of main method is also valid : public
static void main(String... args), and you can access this variable argument as
normal array in Java. E.g. if we use this syntax of main, in your earlier
program we don't need to change anything else, it would work, just like it is
working now.

Why we pass only String object in main method, Can we overload main method to accept other value types as well e.g. Integer, Long or Double? I see no reason, Why we can't overload main method but I am eager to hear from you.